The present invention relates to an ozone gas sensing element, detection apparatus, and measurement method.
At present, air pollution by NOx, SPM, and photochemical oxidant occurs, and the influence on the environment is serious. Ozone as a main component of photochemical oxidant is produced by photochemical reaction of a pollutant such as NOx or hydrocarbon emitted by factories, business offices, and vehicles upon irradiation with sunlight, and causes a photochemical smog.
In Japan, air quality standards have been set for, e.g., the photochemical oxidant concentrations of these substances in air. The gas concentration is measured using analytical instruments that utilize ultraviolet absorption in general air monitoring stations at many places. The air quality standard is an average of 60 ppb or less per hour.
In gas concentration measurement using analytical instruments, a small amount of gas at several ppb can be measured. However, this instrument is expensive and requires maintenance. Analytical instrument requires very high power cost, apparatus maintenance cost, and the like. In addition, many restrictions are posed such that a power supply, a standard gas for calibration, and humidity-controlled dedicated room must be ensured.
In order to perform investigation of the gas concentration distribution and evaluation of the influence on the terrestrial environment at high precision, the number of monitoring points must be increased to monitor the environment on a nationwide scale. For this purpose, a demand has arisen for cumulative use of low-cost, compact, and easy-to-use gas sensors or passive measurement methods (or monitoring apparatuses).
To meet this demand, a semiconductor gas sensor, solid electrolyte gas sensor, electrochemical gas sensor, quartz crystal oscillation gas sensor, and the like are widely developed. However, these gas sensors are developed for evaluating a response within a short time, and not for monitoring which requires data accumulation. If accumulation is necessary, the gas sensor must always be operated. The detection limit is sub-ppm (1 ppm or less), and the gas sensor cannot cope with the detection of ozone at the concentration (e.g., about 10 ppb for ozone) in an actual environment. The influence of another gas cannot be ignored in many cases.
Also, a method using a passive sampler is developed for long-term-averaged measurement on the spot, and is not proper for cumulative use. This method suffers problems such that an operator must go to the site and an individual difference occurs in reading color. The interference or disturbance of another gas often poses a problem.
As the passive measurement method, ozone is sampled by a suction pump into a glass bottle cleaned with purified water so as not to mix air. Ozone in water is absorbed in a potassium iodide solution to titrate precipitated iodine. This method requires not only a sample, but also peripheral devices such as a pump and pH adjustment immediately after water sampling. Further, detection processing must be executed.
Conventional gas concentration measurement requires an expensive, bulky apparatus arrangement in order to detect ozone gas at high precision in ppb order in accordance with the air quality standard. Measurement is cumbersome, and ozone gas cannot be easily detected.